Can you burnish plywood miters?

[FS7]

I assume so, but I don't know. The corner desk I'm building needs a support in the corner (two cabinets on either need, but the "middle" needs a hollow pillar as there will be a USB/electrical outlet on top of the desk).

I did have leftover plywood (well, 1/2" MDF and 1/4" ply) in narrow strips that were perfect for this, so I glued up a box pillar (mitered, about 6" x 36"). It turned out pretty well all things considered. The corners aren't perfect and I do have corner trim available (both the tiny "craft" stuff and actual mahogany corner trim) so I can just cover them, but I thought it might look nice if it were smoother. I'm a little worried that the 1/40" veneer would break when being rolled. Has anybody tried this?

[MsNomer]

I have done it successfully, but here's another thought that came to mind—I’m assuming mahogany ply? Rout rabbets into the two edges of the corner the depth of the thickness of the mahogany edging, so the edging sits flush, and therefore smooth, with the ply instead of proud.

[Cooler]

I've done this with big box store oak plywood.  I gently rubbed the round shaft of a screwdriver over the edges.  It closed up nicely.  But it was a very small gap.  If you have a gap of 1/16" that might not work. 

In that case I would use grain filler (meant for re-finishing flooring.  I use Park brand from Home Depot) and rub it into the corners with a rag like shoe polish.  If you don't want to get grain filler on the surfaces, then mask that off before applying the grain filler. 

The flooring grain filler will take stain, is very smooth and dries quickly.  It sands very easily; so sandpaper in the hand (not an electric sander) will get the job done.  You don't want to sand through the veneer.

[FS7]

I thought about doing rabbets, but I did not think that would end well. Either I'd get tearout with a router or the table saw would drift a bit. It certainly could work and would look nice either with mahogany or a contrasting wood (when I sanded through the veneer on the top, I put in a strip of wenge that looks nice). The plywood was not perfectly straight, either from stress (not likely) or just the fact that it's not super strong. I used three box clamps (four corner) and a whole bunch of smaller clamps to close and align other gaps. There aren't 1/16" gaps, maybe 1/32" or smaller, but they're more likely to be misaligned rather than open mostly because clamping to fix one gap might squeeze one edge further out than it should be.

Masking and filling is a good idea. I sanded a bit already and it looks OK as I had to get the glue out that had squeezed under the clamps. Exposed squeeze out was easy to fix, the hidden wasn't. It won't be stained, so that's not a concern. The mahogany ply is a bit lighter than most of the hardwood (sapele) though it's mostly interchangeable.

[jteneyck]

I've rolled the corners on commercial plywood a few times.  I don't like doing it though because you almost always see a radius instead of a crisp corner.  MsNomer's idea of cutting a rabbet and gluing in a piece of solid wood is perfect as long as it fits the style.  Filler is always a poor choice but sometimes still the best option.  It's not a one size fits all solution.  I would pick whatever best fits your situation.  I know from seeing your work that you are a stickler for details and fine workmanship so I wouldn't be surprised to read in a week that you remade the part.  If you do, add some 45° clamping blocks to the corners.  Then you will be able to pull the joints up tight during glue up.  Cutting the corners to 44-1/2° instead of 45 makes it easier for them to close up tight at the tip of the joint, too. 

John

[FS7]

Remaking isn't really an option since I don't have any more wood to use. If I were to remake I'd just use solid wood and not worry about miters.

I sanded a bit more and I might be overthinking this. Here are some pictures:

https://ibb.co/ftmWJS4

https://ibb.co/Xb6CJk0

On the whole part you can see that the corners aren't really visible. On the detail you can see some combination of tearout, veneer chipping, a gap, whatever. I can't get my fingernail in there but it does bug me.

[MsNomer]

Another consideration you haven’t addressed—what kind of wear will these corners get? As is, those corners will be terribly vulnerable.

[FS7]

Another consideration you haven’t addressed—what kind of wear will these corners get?  As is, those corners will be terribly vulnerable.

That is true. At present, other than my wife kicking it (and I don't even think she will be facing that way) they should not be touched much at all.

[Handplanesandmore]

You might not be happy with what you see if you spray some alcohol on the gappy part (assuming you'll coat the piece with oil or stain).

I'd stain/oil the piece and then fill whatever gap that remains with timbermate. Let it sit for a day, and then match the filler with whatever final finish it's on the surface. The matching part can be intimidating but it's a skill worth developing. Trade restorers do that all the time. In 9.9 out 10 times you'd be the only one who knows about or can spot the fix.

By the way, burnishing a corner is a hack promoted by magazines and amateurs like using glue mixed with saw dust as a filler. It works for some people, but miter gaps left wide open equally work for some others too. Only you can decide what fine woodworking means to you. I once did a cabinet door, but wasn't happy with how it turned out. If I had spent 4 hours on making it, I must have spent an equal amount of time on fixing it to my satisfaction. Why not making a new one? I did not have the wood with the grain that I needed. Had I left the door as is, I'd hate the whole build, and not just the door. That's how I define fine woodworking for myself.

Simon

[jteneyck]

I thought of another potential solution.  You could tip the blade on your TS to 45 and cut a dado parallel with the joint about 1/2 way in, then glue in a piece of solid wood.  Then you have two choices.  Either bring it flush and square with a flush trim router bit, or cut it at a 45 until it's just proud of the plywood, or leave it completely untrimmed proud on purpose as a design feature.  Any of those options would eliminate the gaps and make the corner a lot more durable.  If you used a dado blade you could make the "spline" more substantial, like you always intended the corner to look that way.  

John